Mechanisms of resistance to fusarium wilt (Fusarium udum) were investigated in pigeon pea cultivars from Malawi. Wilt-susceptible (Malawi local) and wilt resistant (ICP 9145) plants were stem-inoculated with a spore suspension containing 2 x 10(6)) conidia/ml of the pathogen. Occlusion of a small proportion of infected vessels was observed, but the resistant reaction appeared to depend mainly on rapid phytoalexin synthesis. Four fungitoxic isoflavonoid phytoalexins-hydroxygenistein, genistein, cajanin and cajanol-were isolated from plants 15 days after inoculation. Cajanol was identified as the main antifungal compound. The concentration of cajanol was 329.4 mug/g in the resistant cultivar as against 88.6 mug/g in the susceptible cultivar 15 days after inoculation. Crude extract from the resistant plants sampled at 24 h after inoculation contained 34.8 mug/ml of cajanol. The LD50 value of cajanol for spore germination was determined as 35 mug/ml. The cajanol content of fungus-infected ICP 9145 10 days after inoculation totally inhibited conidial germination of F. udum.
